This invention relates to apparatus for eliminating time base errors in an information signal and, more particularly, to such apparatus which finds ready application in conjunction with a color video signal which is reproduced from a record medium, whereby time base errors which may be present in the chrominance component of the color video signal are eliminated.
Various devices have been developed for the recording and reproduction of signal information, and particularly video signal information which has a composite color video signal. Examples of such devices include the video tape recorder (VTR) wherein one or more rotary heads scan successive parallel skewed tracks across the surface of a magnetic tape for recording and/or reproducing color video signals from such tracks, magnetic sheet recorders wherein a magnetic sheet is used as the record medium upon which color video signals are recorded in parallel record tracks, and a video disc recorder wherein video signals are recorded in a spiral track or in substantially concentric circular tracks and are reproduced from such tracks by magnetic recording/reproducing apparatus, optical apparatus, and the like. The problem of time base errors in the video signals which are reproduced by the above-mentioned devices is generally common to all of such devices; and to facilitate an explanation thereof, the following discussion is directed specifically to VTR's.
In a typical VTR, the luminance and chrominance components are separated from the color video signal and are processed in separate channels wherein the luminance signal is frequency modulated to a relatively high frequency band while the chrominance component is frequency-converted to a lower frequency band which is below the FM luminance band. These separately processed components then are recombined and recorded simultaneously in successive, skewed record tracks. During a signal reproduction operation, the FM luminance and frequency-converted chrominance components are separated from the reproduced color video signal and are respectively demodulated and frequency reconverted back to their original frequency bands. Then, the recovered luminance and chrominance components are recombined to form the composite color video signal.
In general, the reproduced color video signals contain time base errors, so-called jitter, due to mechanical vibrations in the tape transport mechanism, fluctuations in the rotation of the heads, errors in the speed at which the tape is transported, stretching or shrinkage of the tape since the video signals had been recorded thereon, and other parameters and conditions which will cause time base errors. While these errors typically have only a negligible effect upon the luminance component of the reproduced video signal, they have a particularly serious effect upon the chrominance component, whereby the hue of the reproduced color television picture may be distorted.
One proposal for eliminating or cancelling such time base errors from the reproduced chrominance component provides an automatic frequency control (AFC) circuit for controlling the frequency of the frequency re-converting carrier which is used in the chrominance channel to reconvert the carrier of the chrominance component back to its original frequency. In this AFC circuit, frequency errors in the reproduced chrominance component are detected, and a variable oscillator, such as a voltage-controlled oscillator (VCO) is adjusted to provide an oscillating signal, from which the frequency re-converting signal is derived, so as to cancel the frequency error. The frequency error is deteched by comparing the frequency of the reproduced horizontal synchronizing signals with the frequency of the VCO output. While this AFC operation generally is effective to eliminate relatively large-scale time base errors, such as those time-base errors which influence the frequency of the reproduced horizontal synchronizing signal, the AFC operation has no effect upon relatively small time base errors which may occur in those line intervals which occur between successive horizontal synchronizing signals.
The aforementioned relatively small time base errors appear as fluctuations in the subcarrier of the chrominance component upon which the color information signals are modulated. In one proposal for eliminating such relatively small time base errors, an automatic phase controlled (APC) circuit is used to detect phase errors in the reproduced chrominance subcarrier. In this APC circuit, a highly stable reference carrier is generated and the phase of the reproduced chrominance subcarrier is compared to this reference carrier. In a typical embodiment, the APC circuit is used in conjunction with the AFC circuit, the latter including the aforedescribed VCO circuit, and any phase error which is detected by the APC circuit is used to control the VCO circuit so as to adjust the frequency reconverting signal derived therefrom, whereby such phase errors are eliminated. The reason for providing the APC circuit in conjunction with AFC circuit is to provide adjustments in the frequency reconverting signal in the event that the time base errors which are present in the reproduced color video signal exceed the limited lock-in range of the APC circuit.
In one embodiment of a time base error correcting circuit which uses both an AFC circuit and an APC circuit (known as an AFPC circuit), such as the AFPC described in U.S. Pat. No. 3,723,638, the frequency error control signal which is obtained by detecting the time base frequency error and the phase error control signal which is obtained by detecting the time base phase error are added, and the resultant control signal is used to control the VCO. However, there may be some instances when the frequency and phase error control signals tend to attain opposite adjustments in the frequency reconverting signal. This impedes rapid and satisfactory cancellation or elimination of the time base errors.
The problem of oppositely acting control signals in time base error correcting apparatus is addressed in copending application Ser. No. 851,408 entitled "Time Base Error Correcting Apparatus" filed Nov. 14, 1977 by Kuniyoshi et al. In this copending application, the VCO is controlled with the frequency error signal produced by the AFC circuit only when the time base error in the reproduced video signal is beyond the lock-in range of the APC circuit. The present invention is directed to another technique for solving the problem of oppositely acting frequency and phase error signals in an AFPC circuit.